Non-aqueous electrolyte secondary batteries of high energy density have widely been used with the cordless and portable trend of AV equipment, personal computers, and other electronic apparatuses. Especially lithium secondary batteries are most advanced in practical use. The non-aqueous electrolyte secondary battery has a high electromotive force of approximately 4 V and a high energy density exceeding 350 Wh/L.
Available examples of the non-aqueous electrolyte secondary battery include cylindrical batteries and rectangular batteries. In the cylindrical batteries, a positive electrode plate and a negative electrode plate are wound via a separator and are accommodated, together with a non-aqueous electrolyte in a cylindrical case. In the rectangular batteries, an electrode plate assembly wound in a flat-sided shape is accommodated in a thin rectangular case.
Polymer secondary batteries have recently been in practical use. In the polymer secondary batteries, a stack of electrode plates obtained by interposing a polymer electrolyte between adjoining electrode plates is wrapped with a laminate sheet of a resin film and a metal foil. A gel electrolyte including a non-aqueous liquid electrolyte kept in a polymer matrix is applied for the polymer electrolyte.
The non-aqueous electrolyte secondary battery has high electromotive force, so that the non-aqueous solvent in the electrolyte is readily decomposed. Decomposition of the non-aqueous solvent causes generation of gas, for example, CH4, C2H4, C2H6, CO, CO2, and H2, inside the battery. Methane and carbon dioxide are primary components of the gas.
The production of the gas is accelerated when the battery is kept at high temperatures for a long time period, is used at high temperatures, or is overcharged. The produced gas raises the internal pressure of the battery and may deform or damage the case. The produced gas also accelerates deterioration of the battery characteristics. Especially in the case of the polymer secondary battery, the bulge due to the produced gas causes the polymer electrolyte to be peeled off the electrode plate, and deteriorates the characteristics to the fatal level.
By taking into account generation of the gas due to decomposition of the non-aqueous solvent, the battery is provided with a safety valve that is activated at a preset pressure or a safety mechanism that detects the pressure and cuts off the electric current. An increase in internal pressure of the battery, however, causes frequent activation of the safety valve, which leads to release of the components of the electrolyte as well as the gas and thereby badly affects electronic apparatuses. The high working pressure of the safety valve, on the other hand, leads to easy deformation of a battery case.
In the non-aqueous electrolyte secondary battery containing the non-aqueous solvent, decomposition of the non-aqueous solvent is inevitable. Means for solving the above problems have thus been highly demanded. The following techniques have been proposed to control an increase in internal pressure of the battery due to the produced gas:
(i) Japanese laid-open patent publication No. 6-267593 discloses a battery including a substance capable of absorbing the produced gas or a substance reacting with the gas. This also discloses a positive electrode and a negative electrode with such a substance applied on the surface thereof, as well as a separator with such a substance contained therein.
(ii) Japanese laid-open patent publication No. 11-191400 discloses a multi-layered battery having gas blocking property and rigidity. This battery has a plastic inner housing, and includes a moisture absorbent or a gas absorbent, for example, any of silica gel, zeolite, active carbon, metal salts like stearates, hydrosulfites, and hydrogen absorbing alloys.
(iii) Japanese laid-open patent publication No. 9-180760 shows a mechanism of making the gas produced inside the battery, for example, hydrogen, methane, ethane, and carbon monoxide, electrochemically react with oxides or Ketchen Black added to the electrode plates.
(iv) Japanese laid-open patent publication No. 11-224670 describes that carbon materials, such as active carbon or carbon black have a capacity of absorbing carbon dioxide, carbon monoxide, nitrogen, and argon.
(v) Japanese laid-open patent publication No. 11-54154 discloses a battery including an alkali earth metal oxide (for example, SrO, CaO, BaO, or MgO) for fixation of carbon dioxide. Any of these oxides may be used in a powdery form or as a molded article.
(vi) Japanese laid-open patent publication No. 2000-90971 discloses a positive electrode including active carbon as a gas absorbent and a lithium-containing transition metal oxide.
As described above, diverse efforts have been made to control the increasing internal pressure of the battery due to the produced gas and prevent the resulting decrease in reliability in the non-aqueous electrolyte secondary battery.
The prior art technique, however, does not attain long-term stable control of the increasing internal pressure of the battery. This is because the conventional gas absorbent is readily wetted (excessively wetted) with the non-aqueous solvent of the non-aqueous electrolyte. The excessive wetting of the conventional gas absorbent with the non-aqueous solvent extremely lowers the capacity of gas absorption. Functional groups, such as carbonyl group, carboxyl group, aldehydes group, and hydroxide group are present on the surface of active carbon and carbon black and are expected to accelerate the wetting. Active carbon, for example, is manufactured by firing natural fiber material or synthetic fiber material at relatively low temperatures of 350 to 650° C., which suppress crystallization of carbon, and reforming (activating) the fired fiber material with an acid, an alkali, steam, or zinc chloride. The reforming process increases the surface area of the carbon material, while forming a large number of the functional groups.
Any of the conventional gas absorbents has poor capacity of absorbing methane and carbon dioxide. Since methane and carbon dioxide are the primary components of the gas produced in the battery, only the material having sufficient capacity of absorbing these gases can effectively prevent an increase in internal pressure of the battery.
The battery is generally manufactured in the air or nitrogen, so that the gas absorbent is sealed in the battery after absorption of the air or nitrogen to its saturated level. The gas absorbent that has already absorbed a large quantity of the air or nitrogen can not sufficiently absorb the gas produced in the battery. From the viewpoint of attaining the enhanced productivity and the less scattering and loss of organic solvents, however, it is extremely difficult to manufacture the battery under reduced pressure, with a view to preventing the gas absorbent from absorbing the air or nitrogen.